1. Field of the Invention
This invention relates to apparatus for establishing the various sizes of impactor to be used when impacting bone chips in the intramedullary canal of a femur into which the stem of a hip prosthesis is to be located.
2. Description of the Prior Art
The intramedullary canal at the proximal end of a femur is tapered and when a femoral hip prosthesis is to be fitted, the stem is usually reamed out to accept it. In the technique of using bone chips to surround and locate the stem, the bone chips have first to be impacted. Thus, impactors of different shape and diameter have to be used for different parts of the canal. As considerable force is applied to the bone chips to establish them in place, care must be taken to avoid damaging the walls of the intramedullary canal during the impaction technique.
Impactors comprise an impaction head and a stem which can be attached to an impaction tool but before chips are applied, an intramedullary plug is usually passed down the canal to a desired depth. It is important to establish the distance above the bone plug and each size of impactor can be passed without jamming against the wall of the canal. Driving the impactors beyond this point runs the risk of splitting the femur. It is therefore necessary to check that the impactor which corresponds to one size smaller than the intramedullary plug can be passed over a guide wire down to the plug without obstruction, then taking each larger size impactor in turn, passing it over the wire and noting the depth of insertion to which each will go before it jams against the walls of the canal. This can be done by using an impaction tool which is provided with a gauge from which the depth of insertion can be noted. This reading of the appropriate distance above the plug can be noted for each size of impactor. Subsequently, when impacting the bone chips, the impactor must not be driven beyond the noted depth. The plug depth is read off a marked guide wire which extends into the handle of the impactor and can be read through an appropriate viewing window.
As mentioned above, the impaction depth must be noted on a proforma which must then subsequently be consulted for each size of impactor.
Such impactors are shown in U.S. Pat. Nos. 5,192,283 and 5,470,336.